Better CLI documentation

[brycegbrazen]

It'd be great if we could improve the CLI documentation to be more clear on what the options are, what valid values are for each option are, and several examples using different combinations of the options.

We should make sure to cover the complex cases like the -c option for rez-env where they can be specified with a keyword -c or just as the final positional argument to rez-env.

Motivation
Users having a easier time using the CLI, and less spam in Slack.

[JeanChristopheMorinPerso]

Thanks @brycegbrazen! I think we could potentially look at the pip docs (https://pip.pypa.io/en/stable/cli/pip_install/) to take some inspirations. I like documentation.

Some of it will probably require us to review/re-think how we store/generate the documentation for the commands. In pip's case, the "usage" and "options" sections are automatically generated from the code (like we do), but the "description" section (and its subsections and the "examples" section are right in the sphinx docs.

Do you think something like pip would work for us? Do you have command lines docs that you like and you think we could use as references?